Metal-medium composite sub-wavelength array and gain medium assisted electric field enhancement substrate design

Abstract

A metal-medium composite sub-wavelength array and gain medium assisted electric field enhanced substrate design is established based on CST software, a periodic array transmission enhanced structure is solved by adopting an FEM finite element method and an FDTD time domain finite difference method, two circular ring structures are combined, the circular ring structures are changed into double C-shaped structures which are symmetrically placed, and the array transmission enhanced structure is formed. The conduction electrons are gathered to the middle position of the c-shaped ring hole; and the electric field is further effectively enhanced. The method provides theoretical guidance and experimental basis for amplification of electromagnetic signals and further enhancement of a local field, and has important academic value and practical significance.

Description

Technical Field

[0001] The present invention relates to a design method of a metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate, and belongs to the field of terahertz wave devices. Background Art

[0002] Terahertz (THz) waves are electromagnetic waves with frequencies between 0.1THz and 10THz, a frequency range between microwaves and infrared light. Therefore, they are also called "radiation gaps" or "THz radiation." With the development of materials science, semiconductor technology, and optoelectronics, the application of THz waves is expected to be more widely promoted and applied in the future.

[0003] The role of a gain medium in field enhancement is to boost the intensity of the electromagnetic field. This is particularly true in applications such as lasers and fiber amplifiers. Through stimulated emission and light-matter interactions, it enhances optical signals of specific wavelengths, thereby achieving signal amplification and boosting. The primary function of a gain medium is to amplify light through internal energy level transitions and particle excitations, a process that can significantly enhance the electromagnetic field intensity in a localized region. Gain media are typically laser media, including materials such as solids, gases, liquids, and semiconductors. These materials can produce stimulated emission when excited by a specific pump source (such as light or electricity). This process is one of the core functions of a gain medium, achieving field enhancement through the following mechanisms: stimulated emission, the amplification effect of the gain medium, stimulated emission, and field enhancement. The specific contribution of a gain medium to field enhancement is localized electromagnetic field enhancement. Gain media can locally enhance the electromagnetic field through strong photon-matter interactions. For example, in surface plasmons (SPPs), when light interacts with free electrons on a metal surface, a gain medium (such as an optical fiber or laser dye) can enhance the local electric field, thereby boosting the optical field intensity near the surface. This enhancement effect is of great significance for applications such as biosensing and Raman spectroscopy. However, traditional metasurfaces suffer from low field enhancement factors and low local field enhancement in the terahertz band. This paper proposes a metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate design to address these issues.

[0004] This invention addresses the problem of traditional ring-shaped structures having a low electric field concentration capability and being unable to generate high electric fields. To increase the local near-field enhancement factor in the terahertz band, we considered replacing the circular ring structure with a C-shaped ring hole structure. This structure allows conduction electrons to concentrate in the center of the C-shaped ring hole. Furthermore, by utilizing the slit effect, we symmetrically arranged two C-shaped ring holes to further effectively enhance the electric field. Figure 3(a) shows the relationship between the electric field enhancement factor (EFF) and frequency at the corresponding resonant frequencies for a C-shaped annular aperture array with four different radii, when gap = 2 μm. The figure shows that the highest EFF reaches 70 when the annular aperture radius R = 7 μm and r = 5 μm. Figure 3 (b) shows the relationship between the local electric field enhancement factor value generated by the excitation at the corresponding LSPR peak frequency and frequency when R = 7um, r = 5um, and the slit gap = 2um. It is observed that as the gap increases, the electric field enhancement factor value decreases. Due to structural limitations, the minimum slit gap cannot be less than the ring width, so the maximum electric field enhancement factor value in this structure appears at gap = 2um and reaches 70. To further improve the electric field enhancement capability of our substrate structure, we consider filling the middle of the double C ring hole with a gain medium (n-ik). Adding a gain medium to the gap ring hole can compensate for the absorption loss of the metal material to the optical signal, thereby achieving optical signal amplification and further local field enhancement. This structure combines the advantages of the gain medium with the double C-type slit structure to enhance the local field, further achieving a significant enhancement of the local field in the terahertz band. Figure 3 (a) shows the relationship between the gain coefficient k and the electric field enhancement factor |E| / |E0| at the corresponding resonant wavelength for gain media with different complex refractive indices when R=7um, r=5um, and gap=2um. Figure 3 (b) As can be seen, the structure produces an optimal electric field enhancement factor (EFF) approaching 1000 when the complex refractive index of the gain medium varies with the gain medium's complex refractive index between 1.3 and 0.05i. This represents a factor of more than ten compared to the case without the gain material. The combination of the slit effect and the gain medium significantly enhances the localized electric field in the terahertz band, demonstrating the potential for significant applications of the proposed structure. Summary of the Invention

[0005] The technical problem solved by the present invention is: in order to address the shortcomings of the existing terahertz band metasurfaces in terms of low field enhancement factor and local field enhancement multiple, and the low electric field concentration ability in the structure, which cannot excite a higher electric field, a method for studying a terahertz band double-C-type structure electric field enhancement metasurface is provided. By setting the bottom as a metal total reflection plate, the two symmetrical C-type structures are merged and symmetrically arranged, so that the conduction electrons are concentrated in the middle position of the C-type ring hole, and the electric field is further effectively enhanced.

[0006] The technical solution of the present invention is: a metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate design, including a metamaterial structural unit, such as Figure 1As shown, each metasurface structure is a three-layer structure. The metamaterial structure symmetric units include: the outer radius of the circular hole in the first layer of metal micro-nano structure unit is 7 microns, the inner radius of the circular hole is 5 microns, the radius of the metal column in the metal micro-nano structure unit is a symmetrically placed C-shaped ring hole structure, the metal micro-nano structure layer material is metallic silver, and the dispersion dielectric constant is obtained by Drude model calculation. ω p is the silver plasma frequency, Γ0 is the damping constant; the material of the second dielectric layer is polyimide, the real part of the dielectric constant of the intermediate dielectric layer is 3.5, the imaginary part is 0.009, and the thickness of the intermediate dielectric layer is 5 microns. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 Schematic diagram of the electric field enhancement substrate structure

[0008] Figure 2 (a) Diffraction spectrum (b) XOY cross section and (c) XOZ cross section electric field intensity distribution

[0009] Figure 3 The electric field enhancement factor values ​​under different structural parameters (a) Ring radius R, r change (b) Slit gap change

[0010] Figure 4 (a) Relationship between gain coefficient k and electric field enhancement factor |E| / |E0| at resonant frequency for different n conditions (b) Electric field distribution diagram for several refractive indices

[0011] Figure 5 The graph shows the local electric field enhancement factor value when the refractive index of the gain material is 1.3-0.05i.

[0012] (a) Change R, r when gap = 2um, (b) R = 7um, r = 5um, change gap DETAILED DESCRIPTION

[0013] Verification Process: The electromagnetic simulation software COMSOL Multiphysics 6.2 was used to verify the performance of the proposed dual-C-shaped metasurface, which significantly enhances the localized electric field in the terahertz band. Step 1: COMSOL Multiphysics 6.2 provides a time-domain solver based on the finite element method (FEM), enabling numerical simulation of the metamaterial. The simulation conditions were basically set as periodic boundary conditions in the X and Y directions to simulate the periodic structure in those directions; a perfectly matched layer (PML) was set in the Z direction. The electromagnetic wave was incident along the negative Z axis, with the polarization direction of the linearly polarized TE (TM) wave parallel to the X (Y) axis. The slit effect refers to the generation of a series of secondary wave sources after a light wave (such as monochromatic light) passes through a small slit, according to the Huygens-Fresnel principle. These secondary wave sources propagate outward as waves, causing the light wave to bend and diffuse after the slit. The wave nature becomes more pronounced when the slit size is compared to the wavelength of the incident light. A gain material is also proposed, whose complex refractive index is 1.3-0.05i. The formula for calculating the transmission rate of the metamaterial is where T(f) is the normalized transmissivity as a function of frequency, P(f) is the Poynting vector, and dS is the surface normal direction.

[0014] Step 1: Simulate the transmission performance of the device. The incident electromagnetic wave is a TE wave with a polarization direction parallel to the X-axis. It enters the metamaterial along the negative direction of the Z-axis. The background refractive index is set to a vacuum state, and the gain medium (n-ik) is filled in the middle of the double C-ring hole.

[0015] The second step was to verify the impact of the structural geometric parameters on performance. While holding other structural parameters constant, they varied individual parameters, such as adjusting the radius of the C-shaped ring structure and observing the local electric field enhancement factor. This altered the gap width between the metal slits, affecting the generation of localized surface plasmon resonances between the metal slits and thus enhancing transmission. By manipulating other structural geometric parameters, they observed the impact of changes in the artificial surface plasmon coupling mode on the electric field enhancement, ultimately determining the structural geometric parameters for optimal coupling.

[0016] The embodiment described above is only a preferred solution of the present invention, but is not intended to limit the present invention. In this specification, the preferred embodiment of the present invention disclosed above is only used to help explain the present invention. According to the content of this specification, many similar pattern replacements can be made on the micro-nano structure. The present invention selects and specifically describes this embodiment in order to better explain the principles and practical applications of the invention. Ordinary technicians in the relevant technical field can also make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all technical solutions obtained by equivalent replacement or equivalent transformation fall within the scope of protection of the present invention.

Claims

1. A metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate design. The device comprises several metamaterial units, each of which is a periodically arranged transmission-enhancing device. Each metamaterial unit has a three-layer structure, and the symmetrical units of the metamaterial structure, from top to bottom, include: The outer radius of the circular hole in the first metal micro-nano structure unit is 7 microns, and the inner radius of the circular hole is 5 microns. The radius of the metal pillar in the metal micro-nano structure unit is a symmetrically placed C-shaped ring hole structure. The second dielectric layer is made of polyimide.

2. The metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate design according to claim 1, characterized in that: The first and third dielectric film layers are both made of polyimide, with a real part of a relative dielectric constant of 3.5 and an imaginary part of 0.

009.

3. The metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate design as claimed in claim 1, characterized in that: The material of the second layer in the middle of the metal micro-nano structure layer is silver, and the dielectric constant of silver is calculated by the Drude model.

4. The metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate design as claimed in claim 1, characterized in that: The period (Px) of the first and third layers of the dielectric film layer in the x-direction is 40 microns, the period (Py) in the y-direction is 30 microns, and the thickness (d) is 0.5 microns; the period (Px) of the dielectric intermediate layer is 40 microns, the period (Py) is 30 microns, and the thickness (d) is 5 microns.

5. The metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate design as claimed in claim 1, characterized in that: The complex refractive index of the metal micro-nanostructure layer gain medium is 1.3-0.05i.

6. The metal-dielectric composite subwavelength array combined with a gain medium-assisted electric field enhancement substrate design as claimed in claim 1, characterized in that The outer radius R of the C-shaped ring structure is 7 microns, and the inner radius r is 5 microns.

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